The disabled tabs don't look like disabled in Classic render mode. 

This issue is not observable with the 2016 release as shown in the attached screenshot.

Workarounds:
1) You can change the color of the disabled items by applying the following class to the page:

        .rtsLink.rtsDisabled {
            color: #a5a5a5 !important;
        }

2) Another option is to switch the rendering to Lightweight -> RenderMode="Lightweight".